1. Field of the Invention
The present invention relates to an inverter for driving a lamp of a liquid crystal display (LCD) backlight, more particularly, capable of protecting itself from an overvoltage which, if applied, may damage the lamp or shorten the useful life thereof.
2. Description of the Related Art
In general, a liquid crystal display (LCD) does not generate light on its own, therefore requiring a backlight for irradiating light onto an LCD panel. Until now, a large LCD backlight has mainly employed a cold cathode fluorescent lamp (CCFL) and also includes an inverter for driving the lamp. Notably, it is of great importance to maintain uniform brightness of the lamp of the LCD backlight. To this end, the inverter employs a circuit for feeding back a current of the lamp and keeping the current uniform. Also, the inverter employs an overvoltage protection circuit for protecting the lamp and the inverter circuit in a case where an excessive voltage is applied to the lamp.
FIG. 1 is a circuit diagram illustrating a conventional LCD backlight inverter including a lamp current feedback circuit and an overvoltage protection circuit.
As shown in FIG. 1, the conventional LCD backlight inverter includes an error amplifying unit 11 and a lamp control pulse generator 12. The error amplifying unit 11 receives a voltage corresponding to a current from the lamp (not shown) as a feedback and compares the fed back voltage with a preset first reference voltage Vref1 and outputs an error voltage corresponding to an error therebetween. The lamp control pulse generator 12 outputs a pulse signal having a duty controlled according to an output voltage of the error amplifying unit 11. In addition, the conventional LCD backlight inverter includes an overvoltage protector 13 which receives a voltage corresponding to a voltage applied to the lamp (not shown) as a feedback, compares the fed back voltage with a preset second reference voltage Vref2 and outputs an overvoltage protection signal 13 when the lamp current is greater than the second reference voltage Vref2.
The error amplifying unit 11 includes an error amplifier 112 and a capacitor 113. The error amplifier 112 receives the fed back voltage corresponding to the lamp current through an inverse input terminal and receives the first reference voltage Vref1 through a non-inverse input terminal. The capacitor 113 is connected between the inverse input terminal and an output terminal of the error amplifier 112. Also, the lamp control pulse generator 12 includes a triangle wave oscillator 121 and a first comparator 122. The triangle wave oscillator 121 generates a triangle wave with a predetermined frequency. The first comparator 122 compares the triangle wave generated from the triangle wave oscillator 121 with an output of the error amplifying unit 11 and generates a pulse signal having a duty determined. Moreover, the overvoltage protector 13 includes a second comparator 132 comparing the fed back voltage corresponding to the voltage applied to the lamp with the second preset reference voltage Vref2.
In the conventional LCD backlight inverter having the circuit structure as described above, the error amplifying unit 11 controls the fed back voltage corresponding to the lamp current to be equal to the first reference voltage Vref1. In turn, the lamp control pulse generator 12 generates a pulse signal having a duty determined, thereby controlling the current supplied to the lamp to be uniform. Furthermore, when an overvoltage is applied, that is, the lamp is open, the overvoltage protector 13 determines whether the overvoltage is applied and outputs the overvoltage protection signal which interrupts the current supplied to the lamp.
In the conventional LCD backlight inverter capable of protecting itself from the overvoltage as described above, even in a case where an excessive voltage is applied to the lamp, a pulse signal for controlling the lamp is provided to the lamp through the error amplifying unit 11 and the lamp control pulse generator 12, as in a normal condition. Generally, the lamp has a voltage specification required in an abnormal condition such as an open lamp, which is referred to as an open voltage. That is, to greatly extend the useful life of the lamp, the lamp should be applied with the open voltage, i.e., the voltage specification of the lamp in an open lamp condition or in a case where the overvoltage is applied. This accordingly prevents decline in the useful life of the lamp. However, as described above, in the conventional LCD backlight inverter, when the overvoltage is applied, pulse control is performed as in a normal condition until the current supplied to the lamp is completely interrupted and thus the open voltage may not be applied. Therefore, in the conventional LCD backlight inverter, the lamp may be ruined or reduced in useful life when the overvoltage is applied.